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:mod:`re` --- Regular expression operations
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===========================================
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:synopsis: Regular expression operations.
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.. moduleauthor:: Fredrik Lundh <fredrik@pythonware.com>
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.. sectionauthor:: Andrew M. Kuchling <amk@amk.ca>
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This module provides regular expression matching operations similar to
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Both patterns and strings to be searched can be Unicode strings as well as
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8-bit strings. However, Unicode strings and 8-bit strings cannot be mixed:
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that is, you cannot match an Unicode string with a byte pattern or
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vice-versa; similarly, when asking for a substitution, the replacement
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string must be of the same type as both the pattern and the search string.
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Regular expressions use the backslash character (``'\'``) to indicate
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special forms or to allow special characters to be used without invoking
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their special meaning. This collides with Python's usage of the same
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character for the same purpose in string literals; for example, to match
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a literal backslash, one might have to write ``'\\\\'`` as the pattern
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string, because the regular expression must be ``\\``, and each
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backslash must be expressed as ``\\`` inside a regular Python string
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The solution is to use Python's raw string notation for regular expression
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patterns; backslashes are not handled in any special way in a string literal
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prefixed with ``'r'``. So ``r"\n"`` is a two-character string containing
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``'\'`` and ``'n'``, while ``"\n"`` is a one-character string containing a
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newline. Usually patterns will be expressed in Python code using this raw
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It is important to note that most regular expression operations are available as
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module-level functions and methods on
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:ref:`compiled regular expressions <re-objects>`. The functions are shortcuts
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that don't require you to compile a regex object first, but miss some
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fine-tuning parameters.
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Regular Expression Syntax
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-------------------------
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A regular expression (or RE) specifies a set of strings that matches it; the
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functions in this module let you check if a particular string matches a given
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regular expression (or if a given regular expression matches a particular
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string, which comes down to the same thing).
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Regular expressions can be concatenated to form new regular expressions; if *A*
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and *B* are both regular expressions, then *AB* is also a regular expression.
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In general, if a string *p* matches *A* and another string *q* matches *B*, the
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string *pq* will match AB. This holds unless *A* or *B* contain low precedence
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operations; boundary conditions between *A* and *B*; or have numbered group
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references. Thus, complex expressions can easily be constructed from simpler
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primitive expressions like the ones described here. For details of the theory
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and implementation of regular expressions, consult the Friedl book referenced
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above, or almost any textbook about compiler construction.
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A brief explanation of the format of regular expressions follows. For further
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information and a gentler presentation, consult the :ref:`regex-howto`.
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Regular expressions can contain both special and ordinary characters. Most
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ordinary characters, like ``'A'``, ``'a'``, or ``'0'``, are the simplest regular
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expressions; they simply match themselves. You can concatenate ordinary
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characters, so ``last`` matches the string ``'last'``. (In the rest of this
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section, we'll write RE's in ``this special style``, usually without quotes, and
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strings to be matched ``'in single quotes'``.)
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Some characters, like ``'|'`` or ``'('``, are special. Special
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characters either stand for classes of ordinary characters, or affect
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how the regular expressions around them are interpreted. Regular
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expression pattern strings may not contain null bytes, but can specify
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the null byte using a ``\number`` notation such as ``'\x00'``.
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The special characters are:
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(Dot.) In the default mode, this matches any character except a newline. If
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the :const:`DOTALL` flag has been specified, this matches any character
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(Caret.) Matches the start of the string, and in :const:`MULTILINE` mode also
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matches immediately after each newline.
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Matches the end of the string or just before the newline at the end of the
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string, and in :const:`MULTILINE` mode also matches before a newline. ``foo``
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matches both 'foo' and 'foobar', while the regular expression ``foo$`` matches
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only 'foo'. More interestingly, searching for ``foo.$`` in ``'foo1\nfoo2\n'``
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matches 'foo2' normally, but 'foo1' in :const:`MULTILINE` mode; searching for
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a single ``$`` in ``'foo\n'`` will find two (empty) matches: one just before
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the newline, and one at the end of the string.
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Causes the resulting RE to match 0 or more repetitions of the preceding RE, as
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many repetitions as are possible. ``ab*`` will match 'a', 'ab', or 'a' followed
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by any number of 'b's.
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Causes the resulting RE to match 1 or more repetitions of the preceding RE.
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``ab+`` will match 'a' followed by any non-zero number of 'b's; it will not
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Causes the resulting RE to match 0 or 1 repetitions of the preceding RE.
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``ab?`` will match either 'a' or 'ab'.
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``*?``, ``+?``, ``??``
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The ``'*'``, ``'+'``, and ``'?'`` qualifiers are all :dfn:`greedy`; they match
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as much text as possible. Sometimes this behaviour isn't desired; if the RE
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``<.*>`` is matched against ``'<H1>title</H1>'``, it will match the entire
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string, and not just ``'<H1>'``. Adding ``'?'`` after the qualifier makes it
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perform the match in :dfn:`non-greedy` or :dfn:`minimal` fashion; as *few*
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characters as possible will be matched. Using ``.*?`` in the previous
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expression will match only ``'<H1>'``.
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Specifies that exactly *m* copies of the previous RE should be matched; fewer
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matches cause the entire RE not to match. For example, ``a{6}`` will match
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exactly six ``'a'`` characters, but not five.
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Causes the resulting RE to match from *m* to *n* repetitions of the preceding
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RE, attempting to match as many repetitions as possible. For example,
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``a{3,5}`` will match from 3 to 5 ``'a'`` characters. Omitting *m* specifies a
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lower bound of zero, and omitting *n* specifies an infinite upper bound. As an
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example, ``a{4,}b`` will match ``aaaab`` or a thousand ``'a'`` characters
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followed by a ``b``, but not ``aaab``. The comma may not be omitted or the
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modifier would be confused with the previously described form.
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Causes the resulting RE to match from *m* to *n* repetitions of the preceding
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RE, attempting to match as *few* repetitions as possible. This is the
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non-greedy version of the previous qualifier. For example, on the
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6-character string ``'aaaaaa'``, ``a{3,5}`` will match 5 ``'a'`` characters,
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while ``a{3,5}?`` will only match 3 characters.
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Either escapes special characters (permitting you to match characters like
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``'*'``, ``'?'``, and so forth), or signals a special sequence; special
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sequences are discussed below.
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If you're not using a raw string to express the pattern, remember that Python
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also uses the backslash as an escape sequence in string literals; if the escape
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sequence isn't recognized by Python's parser, the backslash and subsequent
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character are included in the resulting string. However, if Python would
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recognize the resulting sequence, the backslash should be repeated twice. This
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is complicated and hard to understand, so it's highly recommended that you use
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raw strings for all but the simplest expressions.
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Used to indicate a set of characters. In a set:
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* Characters can be listed individually, e.g. ``[amk]`` will match ``'a'``,
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* Ranges of characters can be indicated by giving two characters and separating
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them by a ``'-'``, for example ``[a-z]`` will match any lowercase ASCII letter,
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``[0-5][0-9]`` will match all the two-digits numbers from ``00`` to ``59``, and
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``[0-9A-Fa-f]`` will match any hexadecimal digit. If ``-`` is escaped (e.g.
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``[a\-z]``) or if it's placed as the first or last character (e.g. ``[a-]``),
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it will match a literal ``'-'``.
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* Special characters lose their special meaning inside sets. For example,
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``[(+*)]`` will match any of the literal characters ``'('``, ``'+'``,
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* Character classes such as ``\w`` or ``\S`` (defined below) are also accepted
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inside a set, although the characters they match depends on whether
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:const:`ASCII` or :const:`LOCALE` mode is in force.
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* Characters that are not within a range can be matched by :dfn:`complementing`
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the set. If the first character of the set is ``'^'``, all the characters
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that are *not* in the set will be matched. For example, ``[^5]`` will match
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any character except ``'5'``, and ``[^^]`` will match any character except
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``'^'``. ``^`` has no special meaning if it's not the first character in
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* To match a literal ``']'`` inside a set, precede it with a backslash, or
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place it at the beginning of the set. For example, both ``[()[\]{}]`` and
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``[]()[{}]`` will both match a parenthesis.
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``A|B``, where A and B can be arbitrary REs, creates a regular expression that
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will match either A or B. An arbitrary number of REs can be separated by the
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``'|'`` in this way. This can be used inside groups (see below) as well. As
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the target string is scanned, REs separated by ``'|'`` are tried from left to
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right. When one pattern completely matches, that branch is accepted. This means
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that once ``A`` matches, ``B`` will not be tested further, even if it would
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produce a longer overall match. In other words, the ``'|'`` operator is never
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greedy. To match a literal ``'|'``, use ``\|``, or enclose it inside a
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character class, as in ``[|]``.
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Matches whatever regular expression is inside the parentheses, and indicates the
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start and end of a group; the contents of a group can be retrieved after a match
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has been performed, and can be matched later in the string with the ``\number``
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special sequence, described below. To match the literals ``'('`` or ``')'``,
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use ``\(`` or ``\)``, or enclose them inside a character class: ``[(] [)]``.
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This is an extension notation (a ``'?'`` following a ``'('`` is not meaningful
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otherwise). The first character after the ``'?'`` determines what the meaning
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and further syntax of the construct is. Extensions usually do not create a new
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group; ``(?P<name>...)`` is the only exception to this rule. Following are the
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currently supported extensions.
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(One or more letters from the set ``'a'``, ``'i'``, ``'L'``, ``'m'``,
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``'s'``, ``'u'``, ``'x'``.) The group matches the empty string; the
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letters set the corresponding flags: :const:`re.A` (ASCII-only matching),
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:const:`re.I` (ignore case), :const:`re.L` (locale dependent),
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:const:`re.M` (multi-line), :const:`re.S` (dot matches all),
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and :const:`re.X` (verbose), for the entire regular expression. (The
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flags are described in :ref:`contents-of-module-re`.) This
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is useful if you wish to include the flags as part of the regular
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expression, instead of passing a *flag* argument to the
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:func:`re.compile` function.
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Note that the ``(?x)`` flag changes how the expression is parsed. It should be
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used first in the expression string, or after one or more whitespace characters.
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If there are non-whitespace characters before the flag, the results are
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A non-capturing version of regular parentheses. Matches whatever regular
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expression is inside the parentheses, but the substring matched by the group
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*cannot* be retrieved after performing a match or referenced later in the
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Similar to regular parentheses, but the substring matched by the group is
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accessible via the symbolic group name *name*. Group names must be valid
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Python identifiers, and each group name must be defined only once within a
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regular expression. A symbolic group is also a numbered group, just as if
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the group were not named.
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Named groups can be referenced in three contexts. If the pattern is
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``(?P<quote>['"]).*?(?P=quote)`` (i.e. matching a string quoted with either
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single or double quotes):
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+---------------------------------------+----------------------------------+
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| Context of reference to group "quote" | Ways to reference it |
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+=======================================+==================================+
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| in the same pattern itself | * ``(?P=quote)`` (as shown) |
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+---------------------------------------+----------------------------------+
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| when processing match object ``m`` | * ``m.group('quote')`` |
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| | * ``m.end('quote')`` (etc.) |
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+---------------------------------------+----------------------------------+
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| in a string passed to the ``repl`` | * ``\g<quote>`` |
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| argument of ``re.sub()`` | * ``\g<1>`` |
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+---------------------------------------+----------------------------------+
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A backreference to a named group; it matches whatever text was matched by the
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earlier group named *name*.
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A comment; the contents of the parentheses are simply ignored.
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Matches if ``...`` matches next, but doesn't consume any of the string. This is
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called a lookahead assertion. For example, ``Isaac (?=Asimov)`` will match
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``'Isaac '`` only if it's followed by ``'Asimov'``.
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Matches if ``...`` doesn't match next. This is a negative lookahead assertion.
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For example, ``Isaac (?!Asimov)`` will match ``'Isaac '`` only if it's *not*
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followed by ``'Asimov'``.
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Matches if the current position in the string is preceded by a match for ``...``
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that ends at the current position. This is called a :dfn:`positive lookbehind
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assertion`. ``(?<=abc)def`` will find a match in ``abcdef``, since the
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lookbehind will back up 3 characters and check if the contained pattern matches.
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The contained pattern must only match strings of some fixed length, meaning that
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``abc`` or ``a|b`` are allowed, but ``a*`` and ``a{3,4}`` are not. Note that
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patterns which start with positive lookbehind assertions will not match at the
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beginning of the string being searched; you will most likely want to use the
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:func:`search` function rather than the :func:`match` function:
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>>> m = re.search('(?<=abc)def', 'abcdef')
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This example looks for a word following a hyphen:
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>>> m = re.search('(?<=-)\w+', 'spam-egg')
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.. versionchanged: 3.5
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Added support for group references of fixed length.
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Matches if the current position in the string is not preceded by a match for
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``...``. This is called a :dfn:`negative lookbehind assertion`. Similar to
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positive lookbehind assertions, the contained pattern must only match strings of
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some fixed length. Patterns which start with negative lookbehind assertions may
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match at the beginning of the string being searched.
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``(?(id/name)yes-pattern|no-pattern)``
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Will try to match with ``yes-pattern`` if the group with given *id* or
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*name* exists, and with ``no-pattern`` if it doesn't. ``no-pattern`` is
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optional and can be omitted. For example,
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``(<)?(\w+@\w+(?:\.\w+)+)(?(1)>|$)`` is a poor email matching pattern, which
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will match with ``'<user@host.com>'`` as well as ``'user@host.com'``, but
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not with ``'<user@host.com'`` nor ``'user@host.com>'``.
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The special sequences consist of ``'\'`` and a character from the list below.
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If the ordinary character is not on the list, then the resulting RE will match
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the second character. For example, ``\$`` matches the character ``'$'``.
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Matches the contents of the group of the same number. Groups are numbered
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starting from 1. For example, ``(.+) \1`` matches ``'the the'`` or ``'55 55'``,
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but not ``'thethe'`` (note the space after the group). This special sequence
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can only be used to match one of the first 99 groups. If the first digit of
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*number* is 0, or *number* is 3 octal digits long, it will not be interpreted as
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a group match, but as the character with octal value *number*. Inside the
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``'['`` and ``']'`` of a character class, all numeric escapes are treated as
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Matches only at the start of the string.
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Matches the empty string, but only at the beginning or end of a word.
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A word is defined as a sequence of Unicode alphanumeric or underscore
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characters, so the end of a word is indicated by whitespace or a
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non-alphanumeric, non-underscore Unicode character. Note that formally,
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``\b`` is defined as the boundary between a ``\w`` and a ``\W`` character
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(or vice versa), or between ``\w`` and the beginning/end of the string.
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This means that ``r'\bfoo\b'`` matches ``'foo'``, ``'foo.'``, ``'(foo)'``,
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``'bar foo baz'`` but not ``'foobar'`` or ``'foo3'``.
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By default Unicode alphanumerics are the ones used, but this can be changed
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by using the :const:`ASCII` flag. Inside a character range, ``\b``
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represents the backspace character, for compatibility with Python's string
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Matches the empty string, but only when it is *not* at the beginning or end
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of a word. This means that ``r'py\B'`` matches ``'python'``, ``'py3'``,
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``'py2'``, but not ``'py'``, ``'py.'``, or ``'py!'``.
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``\B`` is just the opposite of ``\b``, so word characters are
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Unicode alphanumerics or the underscore, although this can be changed
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by using the :const:`ASCII` flag.
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For Unicode (str) patterns:
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Matches any Unicode decimal digit (that is, any character in
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Unicode character category [Nd]). This includes ``[0-9]``, and
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also many other digit characters. If the :const:`ASCII` flag is
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used only ``[0-9]`` is matched (but the flag affects the entire
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regular expression, so in such cases using an explicit ``[0-9]``
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may be a better choice).
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For 8-bit (bytes) patterns:
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Matches any decimal digit; this is equivalent to ``[0-9]``.
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Matches any character which is not a Unicode decimal digit. This is
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the opposite of ``\d``. If the :const:`ASCII` flag is used this
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becomes the equivalent of ``[^0-9]`` (but the flag affects the entire
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regular expression, so in such cases using an explicit ``[^0-9]`` may
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For Unicode (str) patterns:
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Matches Unicode whitespace characters (which includes
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``[ \t\n\r\f\v]``, and also many other characters, for example the
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non-breaking spaces mandated by typography rules in many
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languages). If the :const:`ASCII` flag is used, only
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``[ \t\n\r\f\v]`` is matched (but the flag affects the entire
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regular expression, so in such cases using an explicit
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``[ \t\n\r\f\v]`` may be a better choice).
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For 8-bit (bytes) patterns:
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Matches characters considered whitespace in the ASCII character set;
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this is equivalent to ``[ \t\n\r\f\v]``.
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Matches any character which is not a Unicode whitespace character. This is
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the opposite of ``\s``. If the :const:`ASCII` flag is used this
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becomes the equivalent of ``[^ \t\n\r\f\v]`` (but the flag affects the entire
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regular expression, so in such cases using an explicit ``[^ \t\n\r\f\v]`` may
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For Unicode (str) patterns:
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Matches Unicode word characters; this includes most characters
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that can be part of a word in any language, as well as numbers and
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the underscore. If the :const:`ASCII` flag is used, only
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``[a-zA-Z0-9_]`` is matched (but the flag affects the entire
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regular expression, so in such cases using an explicit
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``[a-zA-Z0-9_]`` may be a better choice).
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For 8-bit (bytes) patterns:
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Matches characters considered alphanumeric in the ASCII character set;
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this is equivalent to ``[a-zA-Z0-9_]``.
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Matches any character which is not a Unicode word character. This is
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the opposite of ``\w``. If the :const:`ASCII` flag is used this
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becomes the equivalent of ``[^a-zA-Z0-9_]`` (but the flag affects the
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entire regular expression, so in such cases using an explicit
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``[^a-zA-Z0-9_]`` may be a better choice).
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Matches only at the end of the string.
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Most of the standard escapes supported by Python string literals are also
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accepted by the regular expression parser::
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(Note that ``\b`` is used to represent word boundaries, and means "backspace"
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only inside character classes.)
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``'\u'`` and ``'\U'`` escape sequences are only recognized in Unicode
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patterns. In bytes patterns they are not treated specially.
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Octal escapes are included in a limited form. If the first digit is a 0, or if
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there are three octal digits, it is considered an octal escape. Otherwise, it is
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a group reference. As for string literals, octal escapes are always at most
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three digits in length.
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.. versionchanged:: 3.3
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The ``'\u'`` and ``'\U'`` escape sequences have been added.
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.. deprecated-removed:: 3.5 3.6
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Unknown escapes consist of ``'\'`` and ASCII letter now raise a
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deprecation warning and will be forbidden in Python 3.6.
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Mastering Regular Expressions
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Book on regular expressions by Jeffrey Friedl, published by O'Reilly. The
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second edition of the book no longer covers Python at all, but the first
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edition covered writing good regular expression patterns in great detail.
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.. _contents-of-module-re:
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The module defines several functions, constants, and an exception. Some of the
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functions are simplified versions of the full featured methods for compiled
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regular expressions. Most non-trivial applications always use the compiled
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.. function:: compile(pattern, flags=0)
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Compile a regular expression pattern into a regular expression object, which
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can be used for matching using its :func:`~regex.match` and
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:func:`~regex.search` methods, described below.
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The expression's behaviour can be modified by specifying a *flags* value.
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Values can be any of the following variables, combined using bitwise OR (the
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prog = re.compile(pattern)
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result = prog.match(string)
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result = re.match(pattern, string)
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but using :func:`re.compile` and saving the resulting regular expression
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object for reuse is more efficient when the expression will be used several
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times in a single program.
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The compiled versions of the most recent patterns passed to
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:func:`re.compile` and the module-level matching functions are cached, so
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programs that use only a few regular expressions at a time needn't worry
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about compiling regular expressions.
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Make ``\w``, ``\W``, ``\b``, ``\B``, ``\d``, ``\D``, ``\s`` and ``\S``
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perform ASCII-only matching instead of full Unicode matching. This is only
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meaningful for Unicode patterns, and is ignored for byte patterns.
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Note that for backward compatibility, the :const:`re.U` flag still
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exists (as well as its synonym :const:`re.UNICODE` and its embedded
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counterpart ``(?u)``), but these are redundant in Python 3 since
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matches are Unicode by default for strings (and Unicode matching
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isn't allowed for bytes).
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Display debug information about compiled expression.
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Perform case-insensitive matching; expressions like ``[A-Z]`` will match
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lowercase letters, too. This is not affected by the current locale
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and works for Unicode characters as expected.
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Make ``\w``, ``\W``, ``\b``, ``\B``, ``\s`` and ``\S`` dependent on the
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current locale. The use of this flag is discouraged as the locale mechanism
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is very unreliable, and it only handles one "culture" at a time anyway;
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you should use Unicode matching instead, which is the default in Python 3
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for Unicode (str) patterns. This flag makes sense only with bytes patterns.
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.. deprecated-removed:: 3.5 3.6
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Deprecated the use of :const:`re.LOCALE` with string patterns or
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When specified, the pattern character ``'^'`` matches at the beginning of the
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string and at the beginning of each line (immediately following each newline);
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and the pattern character ``'$'`` matches at the end of the string and at the
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end of each line (immediately preceding each newline). By default, ``'^'``
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matches only at the beginning of the string, and ``'$'`` only at the end of the
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string and immediately before the newline (if any) at the end of the string.
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Make the ``'.'`` special character match any character at all, including a
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newline; without this flag, ``'.'`` will match anything *except* a newline.
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This flag allows you to write regular expressions that look nicer and are
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more readable by allowing you to visually separate logical sections of the
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pattern and add comments. Whitespace within the pattern is ignored, except
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when in a character class or when preceded by an unescaped backslash.
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When a line contains a ``#`` that is not in a character class and is not
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preceded by an unescaped backslash, all characters from the leftmost such
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``#`` through the end of the line are ignored.
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This means that the two following regular expression objects that match a
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decimal number are functionally equal::
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a = re.compile(r"""\d + # the integral part
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\. # the decimal point
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\d * # some fractional digits""", re.X)
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b = re.compile(r"\d+\.\d*")
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.. function:: search(pattern, string, flags=0)
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Scan through *string* looking for the first location where the regular expression
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*pattern* produces a match, and return a corresponding :ref:`match object
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<match-objects>`. Return ``None`` if no position in the string matches the
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pattern; note that this is different from finding a zero-length match at some
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.. function:: match(pattern, string, flags=0)
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If zero or more characters at the beginning of *string* match the regular
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expression *pattern*, return a corresponding :ref:`match object
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<match-objects>`. Return ``None`` if the string does not match the pattern;
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note that this is different from a zero-length match.
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Note that even in :const:`MULTILINE` mode, :func:`re.match` will only match
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at the beginning of the string and not at the beginning of each line.
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If you want to locate a match anywhere in *string*, use :func:`search`
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instead (see also :ref:`search-vs-match`).
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.. function:: fullmatch(pattern, string, flags=0)
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If the whole *string* matches the regular expression *pattern*, return a
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corresponding :ref:`match object <match-objects>`. Return ``None`` if the
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string does not match the pattern; note that this is different from a
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.. versionadded:: 3.4
611
.. function:: split(pattern, string, maxsplit=0, flags=0)
613
Split *string* by the occurrences of *pattern*. If capturing parentheses are
614
used in *pattern*, then the text of all groups in the pattern are also returned
615
as part of the resulting list. If *maxsplit* is nonzero, at most *maxsplit*
616
splits occur, and the remainder of the string is returned as the final element
619
>>> re.split('\W+', 'Words, words, words.')
620
['Words', 'words', 'words', '']
621
>>> re.split('(\W+)', 'Words, words, words.')
622
['Words', ', ', 'words', ', ', 'words', '.', '']
623
>>> re.split('\W+', 'Words, words, words.', 1)
624
['Words', 'words, words.']
625
>>> re.split('[a-f]+', '0a3B9', flags=re.IGNORECASE)
628
If there are capturing groups in the separator and it matches at the start of
629
the string, the result will start with an empty string. The same holds for
630
the end of the string:
632
>>> re.split('(\W+)', '...words, words...')
633
['', '...', 'words', ', ', 'words', '...', '']
635
That way, separator components are always found at the same relative
636
indices within the result list.
640
:func:`split` doesn't currently split a string on an empty pattern match.
643
>>> re.split('x*', 'axbc')
646
Even though ``'x*'`` also matches 0 'x' before 'a', between 'b' and 'c',
647
and after 'c', currently these matches are ignored. The correct behavior
648
(i.e. splitting on empty matches too and returning ``['', 'a', 'b', 'c',
649
'']``) will be implemented in future versions of Python, but since this
650
is a backward incompatible change, a :exc:`FutureWarning` will be raised
653
Patterns that can only match empty strings currently never split the
654
string. Since this doesn't match the expected behavior, a
655
:exc:`ValueError` will be raised starting from Python 3.5::
657
>>> re.split("^$", "foo\n\nbar\n", flags=re.M)
658
Traceback (most recent call last):
659
File "<stdin>", line 1, in <module>
661
ValueError: split() requires a non-empty pattern match.
663
.. versionchanged:: 3.1
664
Added the optional flags argument.
666
.. versionchanged:: 3.5
667
Splitting on a pattern that could match an empty string now raises
668
a warning. Patterns that can only match empty strings are now rejected.
670
.. function:: findall(pattern, string, flags=0)
672
Return all non-overlapping matches of *pattern* in *string*, as a list of
673
strings. The *string* is scanned left-to-right, and matches are returned in
674
the order found. If one or more groups are present in the pattern, return a
675
list of groups; this will be a list of tuples if the pattern has more than
676
one group. Empty matches are included in the result unless they touch the
677
beginning of another match.
680
.. function:: finditer(pattern, string, flags=0)
682
Return an :term:`iterator` yielding :ref:`match objects <match-objects>` over
683
all non-overlapping matches for the RE *pattern* in *string*. The *string*
684
is scanned left-to-right, and matches are returned in the order found. Empty
685
matches are included in the result unless they touch the beginning of another
689
.. function:: sub(pattern, repl, string, count=0, flags=0)
691
Return the string obtained by replacing the leftmost non-overlapping occurrences
692
of *pattern* in *string* by the replacement *repl*. If the pattern isn't found,
693
*string* is returned unchanged. *repl* can be a string or a function; if it is
694
a string, any backslash escapes in it are processed. That is, ``\n`` is
695
converted to a single newline character, ``\r`` is converted to a carriage return, and
696
so forth. Unknown escapes such as ``\&`` are left alone. Backreferences, such
697
as ``\6``, are replaced with the substring matched by group 6 in the pattern.
700
>>> re.sub(r'def\s+([a-zA-Z_][a-zA-Z_0-9]*)\s*\(\s*\):',
701
... r'static PyObject*\npy_\1(void)\n{',
703
'static PyObject*\npy_myfunc(void)\n{'
705
If *repl* is a function, it is called for every non-overlapping occurrence of
706
*pattern*. The function takes a single match object argument, and returns the
707
replacement string. For example:
709
>>> def dashrepl(matchobj):
710
... if matchobj.group(0) == '-': return ' '
712
>>> re.sub('-{1,2}', dashrepl, 'pro----gram-files')
714
>>> re.sub(r'\sAND\s', ' & ', 'Baked Beans And Spam', flags=re.IGNORECASE)
717
The pattern may be a string or an RE object.
719
The optional argument *count* is the maximum number of pattern occurrences to be
720
replaced; *count* must be a non-negative integer. If omitted or zero, all
721
occurrences will be replaced. Empty matches for the pattern are replaced only
722
when not adjacent to a previous match, so ``sub('x*', '-', 'abc')`` returns
725
In string-type *repl* arguments, in addition to the character escapes and
726
backreferences described above,
727
``\g<name>`` will use the substring matched by the group named ``name``, as
728
defined by the ``(?P<name>...)`` syntax. ``\g<number>`` uses the corresponding
729
group number; ``\g<2>`` is therefore equivalent to ``\2``, but isn't ambiguous
730
in a replacement such as ``\g<2>0``. ``\20`` would be interpreted as a
731
reference to group 20, not a reference to group 2 followed by the literal
732
character ``'0'``. The backreference ``\g<0>`` substitutes in the entire
733
substring matched by the RE.
735
.. versionchanged:: 3.1
736
Added the optional flags argument.
738
.. versionchanged:: 3.5
739
Unmatched groups are replaced with an empty string.
741
.. deprecated-removed:: 3.5 3.6
742
Unknown escapes consist of ``'\'`` and ASCII letter now raise a
743
deprecation warning and will be forbidden in Python 3.6.
746
.. function:: subn(pattern, repl, string, count=0, flags=0)
748
Perform the same operation as :func:`sub`, but return a tuple ``(new_string,
749
number_of_subs_made)``.
751
.. versionchanged:: 3.1
752
Added the optional flags argument.
754
.. versionchanged:: 3.5
755
Unmatched groups are replaced with an empty string.
758
.. function:: escape(string)
760
Escape all the characters in pattern except ASCII letters, numbers and ``'_'``.
761
This is useful if you want to match an arbitrary literal string that may
762
have regular expression metacharacters in it.
764
.. versionchanged:: 3.3
765
The ``'_'`` character is no longer escaped.
768
.. function:: purge()
770
Clear the regular expression cache.
773
.. exception:: error(msg, pattern=None, pos=None)
775
Exception raised when a string passed to one of the functions here is not a
776
valid regular expression (for example, it might contain unmatched parentheses)
777
or when some other error occurs during compilation or matching. It is never an
778
error if a string contains no match for a pattern. The error instance has
779
the following additional attributes:
783
The unformatted error message.
785
.. attribute:: pattern
787
The regular expression pattern.
791
The index of *pattern* where compilation failed.
793
.. attribute:: lineno
795
The line corresponding to *pos*.
799
The column corresponding to *pos*.
801
.. versionchanged:: 3.5
802
Added additional attributes.
806
Regular Expression Objects
807
--------------------------
809
Compiled regular expression objects support the following methods and
812
.. method:: regex.search(string[, pos[, endpos]])
814
Scan through *string* looking for a location where this regular expression
815
produces a match, and return a corresponding :ref:`match object
816
<match-objects>`. Return ``None`` if no position in the string matches the
817
pattern; note that this is different from finding a zero-length match at some
820
The optional second parameter *pos* gives an index in the string where the
821
search is to start; it defaults to ``0``. This is not completely equivalent to
822
slicing the string; the ``'^'`` pattern character matches at the real beginning
823
of the string and at positions just after a newline, but not necessarily at the
824
index where the search is to start.
826
The optional parameter *endpos* limits how far the string will be searched; it
827
will be as if the string is *endpos* characters long, so only the characters
828
from *pos* to ``endpos - 1`` will be searched for a match. If *endpos* is less
829
than *pos*, no match will be found; otherwise, if *rx* is a compiled regular
830
expression object, ``rx.search(string, 0, 50)`` is equivalent to
831
``rx.search(string[:50], 0)``.
833
>>> pattern = re.compile("d")
834
>>> pattern.search("dog") # Match at index 0
835
<_sre.SRE_Match object; span=(0, 1), match='d'>
836
>>> pattern.search("dog", 1) # No match; search doesn't include the "d"
839
.. method:: regex.match(string[, pos[, endpos]])
841
If zero or more characters at the *beginning* of *string* match this regular
842
expression, return a corresponding :ref:`match object <match-objects>`.
843
Return ``None`` if the string does not match the pattern; note that this is
844
different from a zero-length match.
846
The optional *pos* and *endpos* parameters have the same meaning as for the
847
:meth:`~regex.search` method.
849
>>> pattern = re.compile("o")
850
>>> pattern.match("dog") # No match as "o" is not at the start of "dog".
851
>>> pattern.match("dog", 1) # Match as "o" is the 2nd character of "dog".
852
<_sre.SRE_Match object; span=(1, 2), match='o'>
854
If you want to locate a match anywhere in *string*, use
855
:meth:`~regex.search` instead (see also :ref:`search-vs-match`).
858
.. method:: regex.fullmatch(string[, pos[, endpos]])
860
If the whole *string* matches this regular expression, return a corresponding
861
:ref:`match object <match-objects>`. Return ``None`` if the string does not
862
match the pattern; note that this is different from a zero-length match.
864
The optional *pos* and *endpos* parameters have the same meaning as for the
865
:meth:`~regex.search` method.
867
>>> pattern = re.compile("o[gh]")
868
>>> pattern.fullmatch("dog") # No match as "o" is not at the start of "dog".
869
>>> pattern.fullmatch("ogre") # No match as not the full string matches.
870
>>> pattern.fullmatch("doggie", 1, 3) # Matches within given limits.
871
<_sre.SRE_Match object; span=(1, 3), match='og'>
873
.. versionadded:: 3.4
876
.. method:: regex.split(string, maxsplit=0)
878
Identical to the :func:`split` function, using the compiled pattern.
881
.. method:: regex.findall(string[, pos[, endpos]])
883
Similar to the :func:`findall` function, using the compiled pattern, but
884
also accepts optional *pos* and *endpos* parameters that limit the search
885
region like for :meth:`match`.
888
.. method:: regex.finditer(string[, pos[, endpos]])
890
Similar to the :func:`finditer` function, using the compiled pattern, but
891
also accepts optional *pos* and *endpos* parameters that limit the search
892
region like for :meth:`match`.
895
.. method:: regex.sub(repl, string, count=0)
897
Identical to the :func:`sub` function, using the compiled pattern.
900
.. method:: regex.subn(repl, string, count=0)
902
Identical to the :func:`subn` function, using the compiled pattern.
905
.. attribute:: regex.flags
907
The regex matching flags. This is a combination of the flags given to
908
:func:`.compile`, any ``(?...)`` inline flags in the pattern, and implicit
909
flags such as :data:`UNICODE` if the pattern is a Unicode string.
912
.. attribute:: regex.groups
914
The number of capturing groups in the pattern.
917
.. attribute:: regex.groupindex
919
A dictionary mapping any symbolic group names defined by ``(?P<id>)`` to group
920
numbers. The dictionary is empty if no symbolic groups were used in the
924
.. attribute:: regex.pattern
926
The pattern string from which the RE object was compiled.
934
Match objects always have a boolean value of ``True``.
935
Since :meth:`~regex.match` and :meth:`~regex.search` return ``None``
936
when there is no match, you can test whether there was a match with a simple
939
match = re.search(pattern, string)
943
Match objects support the following methods and attributes:
946
.. method:: match.expand(template)
948
Return the string obtained by doing backslash substitution on the template
949
string *template*, as done by the :meth:`~regex.sub` method.
950
Escapes such as ``\n`` are converted to the appropriate characters,
951
and numeric backreferences (``\1``, ``\2``) and named backreferences
952
(``\g<1>``, ``\g<name>``) are replaced by the contents of the
955
.. versionchanged:: 3.5
956
Unmatched groups are replaced with an empty string.
958
.. method:: match.group([group1, ...])
960
Returns one or more subgroups of the match. If there is a single argument, the
961
result is a single string; if there are multiple arguments, the result is a
962
tuple with one item per argument. Without arguments, *group1* defaults to zero
963
(the whole match is returned). If a *groupN* argument is zero, the corresponding
964
return value is the entire matching string; if it is in the inclusive range
965
[1..99], it is the string matching the corresponding parenthesized group. If a
966
group number is negative or larger than the number of groups defined in the
967
pattern, an :exc:`IndexError` exception is raised. If a group is contained in a
968
part of the pattern that did not match, the corresponding result is ``None``.
969
If a group is contained in a part of the pattern that matched multiple times,
970
the last match is returned.
972
>>> m = re.match(r"(\w+) (\w+)", "Isaac Newton, physicist")
973
>>> m.group(0) # The entire match
975
>>> m.group(1) # The first parenthesized subgroup.
977
>>> m.group(2) # The second parenthesized subgroup.
979
>>> m.group(1, 2) # Multiple arguments give us a tuple.
982
If the regular expression uses the ``(?P<name>...)`` syntax, the *groupN*
983
arguments may also be strings identifying groups by their group name. If a
984
string argument is not used as a group name in the pattern, an :exc:`IndexError`
987
A moderately complicated example:
989
>>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcolm Reynolds")
990
>>> m.group('first_name')
992
>>> m.group('last_name')
995
Named groups can also be referred to by their index:
1002
If a group matches multiple times, only the last match is accessible:
1004
>>> m = re.match(r"(..)+", "a1b2c3") # Matches 3 times.
1005
>>> m.group(1) # Returns only the last match.
1009
.. method:: match.groups(default=None)
1011
Return a tuple containing all the subgroups of the match, from 1 up to however
1012
many groups are in the pattern. The *default* argument is used for groups that
1013
did not participate in the match; it defaults to ``None``.
1017
>>> m = re.match(r"(\d+)\.(\d+)", "24.1632")
1021
If we make the decimal place and everything after it optional, not all groups
1022
might participate in the match. These groups will default to ``None`` unless
1023
the *default* argument is given:
1025
>>> m = re.match(r"(\d+)\.?(\d+)?", "24")
1026
>>> m.groups() # Second group defaults to None.
1028
>>> m.groups('0') # Now, the second group defaults to '0'.
1032
.. method:: match.groupdict(default=None)
1034
Return a dictionary containing all the *named* subgroups of the match, keyed by
1035
the subgroup name. The *default* argument is used for groups that did not
1036
participate in the match; it defaults to ``None``. For example:
1038
>>> m = re.match(r"(?P<first_name>\w+) (?P<last_name>\w+)", "Malcolm Reynolds")
1040
{'first_name': 'Malcolm', 'last_name': 'Reynolds'}
1043
.. method:: match.start([group])
1046
Return the indices of the start and end of the substring matched by *group*;
1047
*group* defaults to zero (meaning the whole matched substring). Return ``-1`` if
1048
*group* exists but did not contribute to the match. For a match object *m*, and
1049
a group *g* that did contribute to the match, the substring matched by group *g*
1050
(equivalent to ``m.group(g)``) is ::
1052
m.string[m.start(g):m.end(g)]
1054
Note that ``m.start(group)`` will equal ``m.end(group)`` if *group* matched a
1055
null string. For example, after ``m = re.search('b(c?)', 'cba')``,
1056
``m.start(0)`` is 1, ``m.end(0)`` is 2, ``m.start(1)`` and ``m.end(1)`` are both
1057
2, and ``m.start(2)`` raises an :exc:`IndexError` exception.
1059
An example that will remove *remove_this* from email addresses:
1061
>>> email = "tony@tiremove_thisger.net"
1062
>>> m = re.search("remove_this", email)
1063
>>> email[:m.start()] + email[m.end():]
1067
.. method:: match.span([group])
1069
For a match *m*, return the 2-tuple ``(m.start(group), m.end(group))``. Note
1070
that if *group* did not contribute to the match, this is ``(-1, -1)``.
1071
*group* defaults to zero, the entire match.
1074
.. attribute:: match.pos
1076
The value of *pos* which was passed to the :meth:`~regex.search` or
1077
:meth:`~regex.match` method of a :ref:`regex object <re-objects>`. This is
1078
the index into the string at which the RE engine started looking for a match.
1081
.. attribute:: match.endpos
1083
The value of *endpos* which was passed to the :meth:`~regex.search` or
1084
:meth:`~regex.match` method of a :ref:`regex object <re-objects>`. This is
1085
the index into the string beyond which the RE engine will not go.
1088
.. attribute:: match.lastindex
1090
The integer index of the last matched capturing group, or ``None`` if no group
1091
was matched at all. For example, the expressions ``(a)b``, ``((a)(b))``, and
1092
``((ab))`` will have ``lastindex == 1`` if applied to the string ``'ab'``, while
1093
the expression ``(a)(b)`` will have ``lastindex == 2``, if applied to the same
1097
.. attribute:: match.lastgroup
1099
The name of the last matched capturing group, or ``None`` if the group didn't
1100
have a name, or if no group was matched at all.
1103
.. attribute:: match.re
1105
The regular expression object whose :meth:`~regex.match` or
1106
:meth:`~regex.search` method produced this match instance.
1109
.. attribute:: match.string
1111
The string passed to :meth:`~regex.match` or :meth:`~regex.search`.
1116
Regular Expression Examples
1117
---------------------------
1123
In this example, we'll use the following helper function to display match
1124
objects a little more gracefully:
1128
def displaymatch(match):
1131
return '<Match: %r, groups=%r>' % (match.group(), match.groups())
1133
Suppose you are writing a poker program where a player's hand is represented as
1134
a 5-character string with each character representing a card, "a" for ace, "k"
1135
for king, "q" for queen, "j" for jack, "t" for 10, and "2" through "9"
1136
representing the card with that value.
1138
To see if a given string is a valid hand, one could do the following:
1140
>>> valid = re.compile(r"^[a2-9tjqk]{5}$")
1141
>>> displaymatch(valid.match("akt5q")) # Valid.
1142
"<Match: 'akt5q', groups=()>"
1143
>>> displaymatch(valid.match("akt5e")) # Invalid.
1144
>>> displaymatch(valid.match("akt")) # Invalid.
1145
>>> displaymatch(valid.match("727ak")) # Valid.
1146
"<Match: '727ak', groups=()>"
1148
That last hand, ``"727ak"``, contained a pair, or two of the same valued cards.
1149
To match this with a regular expression, one could use backreferences as such:
1151
>>> pair = re.compile(r".*(.).*\1")
1152
>>> displaymatch(pair.match("717ak")) # Pair of 7s.
1153
"<Match: '717', groups=('7',)>"
1154
>>> displaymatch(pair.match("718ak")) # No pairs.
1155
>>> displaymatch(pair.match("354aa")) # Pair of aces.
1156
"<Match: '354aa', groups=('a',)>"
1158
To find out what card the pair consists of, one could use the
1159
:meth:`~match.group` method of the match object in the following manner:
1163
>>> pair.match("717ak").group(1)
1166
# Error because re.match() returns None, which doesn't have a group() method:
1167
>>> pair.match("718ak").group(1)
1168
Traceback (most recent call last):
1169
File "<pyshell#23>", line 1, in <module>
1170
re.match(r".*(.).*\1", "718ak").group(1)
1171
AttributeError: 'NoneType' object has no attribute 'group'
1173
>>> pair.match("354aa").group(1)
1180
.. index:: single: scanf()
1182
Python does not currently have an equivalent to :c:func:`scanf`. Regular
1183
expressions are generally more powerful, though also more verbose, than
1184
:c:func:`scanf` format strings. The table below offers some more-or-less
1185
equivalent mappings between :c:func:`scanf` format tokens and regular
1188
+--------------------------------+---------------------------------------------+
1189
| :c:func:`scanf` Token | Regular Expression |
1190
+================================+=============================================+
1192
+--------------------------------+---------------------------------------------+
1193
| ``%5c`` | ``.{5}`` |
1194
+--------------------------------+---------------------------------------------+
1195
| ``%d`` | ``[-+]?\d+`` |
1196
+--------------------------------+---------------------------------------------+
1197
| ``%e``, ``%E``, ``%f``, ``%g`` | ``[-+]?(\d+(\.\d*)?|\.\d+)([eE][-+]?\d+)?`` |
1198
+--------------------------------+---------------------------------------------+
1199
| ``%i`` | ``[-+]?(0[xX][\dA-Fa-f]+|0[0-7]*|\d+)`` |
1200
+--------------------------------+---------------------------------------------+
1201
| ``%o`` | ``[-+]?[0-7]+`` |
1202
+--------------------------------+---------------------------------------------+
1203
| ``%s`` | ``\S+`` |
1204
+--------------------------------+---------------------------------------------+
1205
| ``%u`` | ``\d+`` |
1206
+--------------------------------+---------------------------------------------+
1207
| ``%x``, ``%X`` | ``[-+]?(0[xX])?[\dA-Fa-f]+`` |
1208
+--------------------------------+---------------------------------------------+
1210
To extract the filename and numbers from a string like ::
1212
/usr/sbin/sendmail - 0 errors, 4 warnings
1214
you would use a :c:func:`scanf` format like ::
1216
%s - %d errors, %d warnings
1218
The equivalent regular expression would be ::
1220
(\S+) - (\d+) errors, (\d+) warnings
1223
.. _search-vs-match:
1225
search() vs. match()
1226
^^^^^^^^^^^^^^^^^^^^
1228
.. sectionauthor:: Fred L. Drake, Jr. <fdrake@acm.org>
1230
Python offers two different primitive operations based on regular expressions:
1231
:func:`re.match` checks for a match only at the beginning of the string, while
1232
:func:`re.search` checks for a match anywhere in the string (this is what Perl
1237
>>> re.match("c", "abcdef") # No match
1238
>>> re.search("c", "abcdef") # Match
1239
<_sre.SRE_Match object; span=(2, 3), match='c'>
1241
Regular expressions beginning with ``'^'`` can be used with :func:`search` to
1242
restrict the match at the beginning of the string::
1244
>>> re.match("c", "abcdef") # No match
1245
>>> re.search("^c", "abcdef") # No match
1246
>>> re.search("^a", "abcdef") # Match
1247
<_sre.SRE_Match object; span=(0, 1), match='a'>
1249
Note however that in :const:`MULTILINE` mode :func:`match` only matches at the
1250
beginning of the string, whereas using :func:`search` with a regular expression
1251
beginning with ``'^'`` will match at the beginning of each line.
1253
>>> re.match('X', 'A\nB\nX', re.MULTILINE) # No match
1254
>>> re.search('^X', 'A\nB\nX', re.MULTILINE) # Match
1255
<_sre.SRE_Match object; span=(4, 5), match='X'>
1261
:func:`split` splits a string into a list delimited by the passed pattern. The
1262
method is invaluable for converting textual data into data structures that can be
1263
easily read and modified by Python as demonstrated in the following example that
1264
creates a phonebook.
1266
First, here is the input. Normally it may come from a file, here we are using
1267
triple-quoted string syntax:
1269
>>> text = """Ross McFluff: 834.345.1254 155 Elm Street
1271
... Ronald Heathmore: 892.345.3428 436 Finley Avenue
1272
... Frank Burger: 925.541.7625 662 South Dogwood Way
1275
... Heather Albrecht: 548.326.4584 919 Park Place"""
1277
The entries are separated by one or more newlines. Now we convert the string
1278
into a list with each nonempty line having its own entry:
1281
:options: +NORMALIZE_WHITESPACE
1283
>>> entries = re.split("\n+", text)
1285
['Ross McFluff: 834.345.1254 155 Elm Street',
1286
'Ronald Heathmore: 892.345.3428 436 Finley Avenue',
1287
'Frank Burger: 925.541.7625 662 South Dogwood Way',
1288
'Heather Albrecht: 548.326.4584 919 Park Place']
1290
Finally, split each entry into a list with first name, last name, telephone
1291
number, and address. We use the ``maxsplit`` parameter of :func:`split`
1292
because the address has spaces, our splitting pattern, in it:
1295
:options: +NORMALIZE_WHITESPACE
1297
>>> [re.split(":? ", entry, 3) for entry in entries]
1298
[['Ross', 'McFluff', '834.345.1254', '155 Elm Street'],
1299
['Ronald', 'Heathmore', '892.345.3428', '436 Finley Avenue'],
1300
['Frank', 'Burger', '925.541.7625', '662 South Dogwood Way'],
1301
['Heather', 'Albrecht', '548.326.4584', '919 Park Place']]
1303
The ``:?`` pattern matches the colon after the last name, so that it does not
1304
occur in the result list. With a ``maxsplit`` of ``4``, we could separate the
1305
house number from the street name:
1308
:options: +NORMALIZE_WHITESPACE
1310
>>> [re.split(":? ", entry, 4) for entry in entries]
1311
[['Ross', 'McFluff', '834.345.1254', '155', 'Elm Street'],
1312
['Ronald', 'Heathmore', '892.345.3428', '436', 'Finley Avenue'],
1313
['Frank', 'Burger', '925.541.7625', '662', 'South Dogwood Way'],
1314
['Heather', 'Albrecht', '548.326.4584', '919', 'Park Place']]
1320
:func:`sub` replaces every occurrence of a pattern with a string or the
1321
result of a function. This example demonstrates using :func:`sub` with
1322
a function to "munge" text, or randomize the order of all the characters
1323
in each word of a sentence except for the first and last characters::
1326
... inner_word = list(m.group(2))
1327
... random.shuffle(inner_word)
1328
... return m.group(1) + "".join(inner_word) + m.group(3)
1329
>>> text = "Professor Abdolmalek, please report your absences promptly."
1330
>>> re.sub(r"(\w)(\w+)(\w)", repl, text)
1331
'Poefsrosr Aealmlobdk, pslaee reorpt your abnseces plmrptoy.'
1332
>>> re.sub(r"(\w)(\w+)(\w)", repl, text)
1333
'Pofsroser Aodlambelk, plasee reoprt yuor asnebces potlmrpy.'
1339
:func:`findall` matches *all* occurrences of a pattern, not just the first
1340
one as :func:`search` does. For example, if one was a writer and wanted to
1341
find all of the adverbs in some text, he or she might use :func:`findall` in
1342
the following manner:
1344
>>> text = "He was carefully disguised but captured quickly by police."
1345
>>> re.findall(r"\w+ly", text)
1346
['carefully', 'quickly']
1349
Finding all Adverbs and their Positions
1350
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
1352
If one wants more information about all matches of a pattern than the matched
1353
text, :func:`finditer` is useful as it provides :ref:`match objects
1354
<match-objects>` instead of strings. Continuing with the previous example, if
1355
one was a writer who wanted to find all of the adverbs *and their positions* in
1356
some text, he or she would use :func:`finditer` in the following manner:
1358
>>> text = "He was carefully disguised but captured quickly by police."
1359
>>> for m in re.finditer(r"\w+ly", text):
1360
... print('%02d-%02d: %s' % (m.start(), m.end(), m.group(0)))
1368
Raw string notation (``r"text"``) keeps regular expressions sane. Without it,
1369
every backslash (``'\'``) in a regular expression would have to be prefixed with
1370
another one to escape it. For example, the two following lines of code are
1371
functionally identical:
1373
>>> re.match(r"\W(.)\1\W", " ff ")
1374
<_sre.SRE_Match object; span=(0, 4), match=' ff '>
1375
>>> re.match("\\W(.)\\1\\W", " ff ")
1376
<_sre.SRE_Match object; span=(0, 4), match=' ff '>
1378
When one wants to match a literal backslash, it must be escaped in the regular
1379
expression. With raw string notation, this means ``r"\\"``. Without raw string
1380
notation, one must use ``"\\\\"``, making the following lines of code
1381
functionally identical:
1383
>>> re.match(r"\\", r"\\")
1384
<_sre.SRE_Match object; span=(0, 1), match='\\'>
1385
>>> re.match("\\\\", r"\\")
1386
<_sre.SRE_Match object; span=(0, 1), match='\\'>
1392
A `tokenizer or scanner <http://en.wikipedia.org/wiki/Lexical_analysis>`_
1393
analyzes a string to categorize groups of characters. This is a useful first
1394
step in writing a compiler or interpreter.
1396
The text categories are specified with regular expressions. The technique is
1397
to combine those into a single master regular expression and to loop over
1398
successive matches::
1403
Token = collections.namedtuple('Token', ['typ', 'value', 'line', 'column'])
1406
keywords = {'IF', 'THEN', 'ENDIF', 'FOR', 'NEXT', 'GOSUB', 'RETURN'}
1407
token_specification = [
1408
('NUMBER', r'\d+(\.\d*)?'), # Integer or decimal number
1409
('ASSIGN', r':='), # Assignment operator
1410
('END', r';'), # Statement terminator
1411
('ID', r'[A-Za-z]+'), # Identifiers
1412
('OP', r'[+\-*/]'), # Arithmetic operators
1413
('NEWLINE', r'\n'), # Line endings
1414
('SKIP', r'[ \t]+'), # Skip over spaces and tabs
1415
('MISMATCH',r'.'), # Any other character
1417
tok_regex = '|'.join('(?P<%s>%s)' % pair for pair in token_specification)
1420
for mo in re.finditer(tok_regex, code):
1422
value = mo.group(kind)
1423
if kind == 'NEWLINE':
1424
line_start = mo.end()
1426
elif kind == 'SKIP':
1428
elif kind == 'MISMATCH':
1429
raise RuntimeError('%r unexpected on line %d' % (value, line_num))
1431
if kind == 'ID' and value in keywords:
1433
column = mo.start() - line_start
1434
yield Token(kind, value, line_num, column)
1438
total := total + price * quantity;
1439
tax := price * 0.05;
1443
for token in tokenize(statements):
1446
The tokenizer produces the following output::
1448
Token(typ='IF', value='IF', line=2, column=4)
1449
Token(typ='ID', value='quantity', line=2, column=7)
1450
Token(typ='THEN', value='THEN', line=2, column=16)
1451
Token(typ='ID', value='total', line=3, column=8)
1452
Token(typ='ASSIGN', value=':=', line=3, column=14)
1453
Token(typ='ID', value='total', line=3, column=17)
1454
Token(typ='OP', value='+', line=3, column=23)
1455
Token(typ='ID', value='price', line=3, column=25)
1456
Token(typ='OP', value='*', line=3, column=31)
1457
Token(typ='ID', value='quantity', line=3, column=33)
1458
Token(typ='END', value=';', line=3, column=41)
1459
Token(typ='ID', value='tax', line=4, column=8)
1460
Token(typ='ASSIGN', value=':=', line=4, column=12)
1461
Token(typ='ID', value='price', line=4, column=15)
1462
Token(typ='OP', value='*', line=4, column=21)
1463
Token(typ='NUMBER', value='0.05', line=4, column=23)
1464
Token(typ='END', value=';', line=4, column=27)
1465
Token(typ='ENDIF', value='ENDIF', line=5, column=4)
1466
Token(typ='END', value=';', line=5, column=9)
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removed
python-3.5.1-docs-html/_sources/library/re.txt
